Process for treating polyethylene structures and articles resulting therefrom



- Aug. 1957 L. E. WOLINSKE w PROCESS FOR TREATING POLYETHYLENESTRUCTURES 7 AND ARTICLES RESULTING THEREFROM File Nav. 29, 1952 INVENTO R LEON E. WOLIN-fi'KI ATTORNEY nite J PRUCESS FCR TREATINGPOLYETHYLENE STRUCTURES AND ARTICLES RESULTING THEREFRQM ApplicationNovember 29, 1952, Serial No. 323,275 Claims. (Cl. 184S) E. I. tin DeL,a

This invention relates to a process of treating the surface ofpolyethylene structures and, more particularly, to a process of treatingthe surface of a polyethylene film to promote the adhesion thereto ofprinting inks and various other materials.

U. S. Patent 2,219,700 to Perrin et al. discloses and claims apolyethylene film, i. e., a film of a solid polymer of ethylene. Ingeneral, polyethylene films are tough, semi-transparent, resistant tomany chemicals, exhibit a high degree of moisture vapor impermeability,permit the passage of oxygen, and are heat-sealable. Because of thiscombination of properties, polyethylene films are highly useful forpackaging and wrapping a great variety of materials such as chemicals,fresh produce, dried milk, textiles, hardware, etc. Probably the onlytroublesome dis advantage of polyethylene film for use in the packagingfield is the fact that standard aniline and rotogravure printinginksemployed for printing various cellulosic films, such as regeneratedcellulose and, cellulose acetate films, do not adhere satisfactorily tothe surface of the film. Generally, any indicia, such as trade marks,advertising indicia, recipes, etc, imprinted upon a surface of apolyethylene film with standard oil or lacquer-type inks em- ?Patentployed for printing cellophane film are easily smeared or rubbed off bythe normal abrasions suffered by packages during shipping, handling,etc. Hence, in order to. obtain satisfactory adhesion. betweena driedink and a polyethylene film surface, it is necessary to employ aspecially com.- pounded ink or modify the film surface to promoteimproved ink adhesion.

Although printing inks compounded particularly for printing onpolyethylene films have been developed, the

use. of most of these inks requires modification of standard printing.processes; and the preferred approach is treatment of the polyethylene'film surface to promote. adhesion of standard oil and lacquer typeinks.

An object of the present invention is to provide a process of treatingthe surface of a polyethylene structure, e. g., film, to improveadhesion of standard printing inks, i. e., promote adhesion of standardaniline and rotogravure inks employed in printing on cellophane film.Another object is to provide a process of treating the surface of apolyethylene film to improve adhesion thereof to various othermaterials, such as metals, paper, nitrocellulose coatings, and otherpolymeric coatings, e. g., nylon, polyethylene terephthalate, etc. Afurther object is to improve the adhesion of polyethylene film to itselfand other materials when 'using commercial adhesives. 'A still furtherobject is to provide a process of treating the surface of a polyethylenefilm to improve adhesion thereto of dried ink impressions and not impairthe transparency of the film. A still further object is to provide apolyethylene film having modified surface characteristics such thatdried ink imprints on the surface will not rub off when tested inaccordance with the various tests described hereinafter. Other objectswill be apparent from the following description of the invention.

These objects are realized by the present invention which, brieflystated, comprises subjecting a polyethylene structure, e. g., film, at atemperature within the range of from about 150'to about 325 C. to theaction of ozone, preferably in the presence of ultra-violet light, andthereafter quenching said structure in an aqueous quench bath containinga conditioning agent selected from the group of oxy compounds consistingof hydrogen peroxide, nitrous acid, alkaline hypochlorites, concentratednitric acid, and mixtures of concentrated nitric acid and sulfuric acid.

' It should be understood that 325 C. is not an upper limit insofar asthe operability of the present invention is concerned. Extrusion ofpresently available polyethylene compositions at temperaturessubstantially higher than 325 C. is not practical because the melt istoo fluid, and presently known antioxidants do not efficiently preventdegradation of the polymer at appreciably higher temperatures. With thedevolopment of polyethylene compositions which form a more viscous meltand the discovery of more efficient antioxidants, extrusion may becarried out more rapidly and efiiciently at temperatures of 400 C. andabove, the maximum being that temperature beyond which substantialdegradation of the polyethylene composition occurs.

7 In the normal process of extruding molten polyethylone into film form,a molding powder or flake of polyethylene is fed continuously into amelt extrusion mt.- chine, and the molten film continuously extrudedthrough a slot orifice and through an air gap vertically downward into aquench bath maintained at a temperature from -2595 C., preferably from30-60 C. Usually, the

polyethylene is extruded from a melt maintained at a temperature withinthe range from 150 to 325 C. Tubing'is usually extruded from a melt at atemperature within the range from l-200 C., whereas film is extruded ata temperature within the range from 250- 325 C. An alternative processof forming a polyethylene film comprises milling molten polymer onclosely-spaced calender rolls to form a film which is conductedvertically downward into a quench bath. In either of these generalmethods of forming a polymeric film, the space between the point. wherethe molten film leaves the slot orifice or the last calender roll andthe point where the molten film enters a quench bath will hereinafter betermed the air gap. During passage through the air gap, the film ismerely permitted to pass uninhibited through the atmosphere, and thisprovides for some superficial cooling.

..Generally, the length of the air gap ranges from about 2" to as longas 15" in some cases.

Because of the rapid action of ozone upon the surface of thepolyethylene film at elevated temperatures, the process of the presentinvention is most conveniently carried out by subjecting freshlyextruded film at a temperature of from about to about 325 C. to theaction of ozone as the film passes through the air gap. For example,freshly extruded film may be treated in accordance with this inventionby suitably enclosing the air gap and providing for the maintenance inthe enclosure of a gaseous atmosphere, e. g., air, containing ozone.Provision may be made for the employment of ultraviolet light by makingthe walls of the enclosure transparent to ultra-violet light or byinstalling a source of ultra-violet light inside of an opaque enclosure.Hence, the process of the present invention may be carried out by makingvery simple modifications to existing filmforming or tube-formingapparatus; and, owing to the rapid action of ozone, employment of thepresent process does not preclude production of film at commerciallysatisfactory rates. Normally, in order that polythene film may betreated in accordance with the present invention as part of presentlyemployed extrusion or calendering techniques of forming the film, thetime. of treat ment in the air gap should be no greater than about 2,

seconds in order to permit operation at commercially acceptable rates.Treating chambers longer than the normal air gap spacing, i. e., usually5l0", may be employed to allow for longer treatment with ozone.Futhermore, the path of film travel in the quench bath may be lengthenedto increase the time of treatment with the oxy-compound.

Treatment of the film maintained at an elevated temperature is essentialfor obtaining rapid modification of the film surface with ozone.Normally, treatment at temperatures substantially below 150 C. does notprovide for obtaining rapid action with ozone.

Any conditioning agent selected from the group of oxy compoundsconsisting of hydrogen peroxide, nitrous'acid, alkaline hypochloritcs,concentrated nitric acidand a mixture of concentrated nitric acid andsulfuric acid may be employed in the quench bath for purposes of thisinvention. The conditioning agents may be employed in the quench bath invarying concentrations as illustrated in the examples presentedhereinafter. Hydrogen peroxide has been employed in concentrationsranging from 0.24% to 32%, by weight of the total bath; and higher andsomewhat lower concentrations are entirely operable. The same applies tothe alkaline hypochlorites, but about 30% or higher is preferred. Whennitric acid or mixtures thereof with sulfuric acid are used, theconcentrated acids are preferred. Following the treatment with theseagents, the polyethylene film is passed into an alkaline neutralizingbath, such as aqueous sodium hydroxide; and thereafter the film iswashed in water.

For obtaining the desired physical characteristics in the polyethylenefilm, the quench bath is normally maintained at a temperature between 25and 65 C. However, if the physical properties of the resulting film arenot impaired or reduced below the minimum requirements for particularend uses, the use of higher quench.

bath temperatures, e. g., up to 90-95 C., permits shorter chamber; andadditional ozone is injected into the out gases which are thenrecirculated. Ozone concentrations as high as 10% do not tend to burnthe film, i. e., impair transparency or semi-transparency of the film,so long as the time of treatment or exposure is not excessive.

Preferably, ultra-violet light having a wave length no greater than 3900A. U. is employed to accelerate the action of ozone, particularly wherethe process of this invention is incorporated in the continuousproduction of film.

The following examples illustrate the preferred practice of thisinvention, reference being had to the accompanying drawing wherein isshown diagrammatically an arrangement of apparatus used in carrying outthe process of this invention.

Referring to the drawing, molten polyethylene, at a temperature of 265C., was extruded in the form of a film F from extrusion hopper 1 intothe air gap surrounded by an enclosure constituting a treating chamber2, the walls of which are formed in part, at least, of transparentmaterial (quartz glass) to provide for the transmission therethrough ofultra-violet light emitted from mercury arc lamps 3 placed two inchesfrom the film. The upper end of the chamber was closed by the extrusionhopper, and the bottom of the chamber was sealed from the atmosphere byprojecting the sides thereof below the surface of the cooling liquid (60C.) in the quench bath 4. The length of the air gap was 10 inches, andthe path of travel of the film in the quench bath was also 10 inches.Air at atmospheric pressure and containing ozone was passed into thechamber at 5 and out at 6. At no time did the temperature of film inchamber 2 drop below 200 C. The treatment times set forth in thefollowing table are the actual times that any given increment of thefilm remained in the treating chamber and in the quench bath.

Table Composition of Temp.of Ozone Timein Tlmein Example Quench Bath(Oon- Quench Concen- Air Gap Quench Prlntabillty ditioning Agent) Bath,0. tration, (seconds) Bath Percent (seconds) 0.24:3 ?1 Hydrogen Per- 601 1. 06 1.06 Excellent.

0 e. 3.5% Hydrogen Per- 60 1 1. 01 1. 01 D0.

0 e. 31.8% Hydrogen Per- 1 .98 .98 Do.

oxide. ..do 90 .1 .75 .75 Do. 30% Sodium Hypo- 30 .1 1.06 1.06 Do.

chlorite. do 90 1 .85 .85 Do. Cone. HNO; 1 75 75 Do. Mixed Cone. H1804,60 1 .52 .52 Do.

HNO: 1/8.

treatment with ozone. In general, quench bath temperatures substantiallyhigher than C. do not sutficiently quench the film; and with slowercooling eifected by higher bath temperatures, the polyethylene filmformed is generally more crystalline and, consequently, has lowerstrength properties, lower flexibility and transparency, and the bondstrengths of heat seals are considerably lower. Hence, quenching bathtemperatures should be maintained below 65 C.

For eifective action within the relatively short exposure timespermissible in the treatment of film being extruded at commerciallyacceptable rates, the concentration of ozone in the treating atmospheremust be at least 0.05% by volume, of the total volume of gases presentin the treating gases. The use of ozone concentrations substantiallygreater than 5%, by volume of the total gases surrounding the film, isnot particularly practical because of the restricted capacity ofpresent-day ozone generating equipment. This applies to continuoustreatment of film wherein the ozone-containing gas, e. g., air, ispassed continuously through the treating In evaluating the printability,i. e., the strength of the adhesive bond between the dried ink and thetreated polyethylene film surface, a number of tests were employed (5 inall); and, on the basis of the results of all of the tests, the filmswere rated either acceptable or not acceptable and, if acceptable,either excellent or good. Four diiferent inks were employed to print thetreated surfaces of polyethylene films, and each printed sample wasevaluated in accordance with each of the five tests which will bedescribed hereinafter. The inks employed were as follows:

No. 1'Am'line Cellophane Ink (Bensing Bros. and Deeney, No. W-400).

No. 2'Aniline Polyethylene Ink (Interchemical Corporation, No. PA-Red)No. 3Rotogravure Cellophane Ink (Bensing Bros. and Deeney, No. G-1037).

No. 4-Rotogravure Polyethylene Ink (Interchemical Corporation,IN-Tag-Red, GPA Red).

In preparing the printed samples of polyethylene film, the ink wasapplied with a commercial ink spreader which comprised a steel rodhaving fine wire wrappedaround the rod. The spreader produced amultiplicity of fine lines. The ink was then dried for three minutes at70 C. and thereafter permitted to cool to room temperature. Each samplewas then tested in accordance with each of the following tests, and theamount of ink rubbed off and/ or removed Was noted:

1. Rub test-The inked polyethylene surface was rubbed ten times againsta hard white paper.

2. Scratch test.-The back of a fingernail was rubbed across the inkedsurface.

3. Flex test-The film was held between thumb and forefinger (2" apart)and flexed vigorously.

4. Pressure-sensitive tape test.,A pressure-sensitive tape was pressedagainst the printed surface, and then pulled ofi.

5. Twist test-The printed film was folded once and then again in adirection perpendicular to the first fold. The folded ends were thentwisted once around and thereafter the film surface was examined forsmearing and/ or cracking of the dried ink.

Although the present process is employed primarily for treating thesurface of a polyethylene film in order to produce a film which may besuccessfully printed with standard oil or lacquer type inks, e. g.,aniline or rotogravure inks employed for printing on cellophane film,the present invention may be employed to modify the surface of apolyethylene film which is to be printed with inks which are especiallymodified for printing upon a polyethylene film surface. The net resultis an even further improvement in the adhesive bonds between the driedink and the polyethylene film surface. The present invention furtherprovides for the preparation of a polyethylene film which is morereadily adherent to metals, papers, and various coatings, such as thoseof nitrocellulose; polyamides, e. g., polyhexamethylene adipamide,polyhexamethylene sebacamide, N-methoxymethyl polyhexamethyleneadipamide and other polyamides defined in U. S. P. 2,430,860, andinterpolyamides defined in U. S. P. 2,285,009; polyethyleneterephthalate; polyvinyl acetals such as polyvinyl butyral; ethylcellulose; vinyl acetate-vinyl chloride copolymers; vinylidene chloridecopolymers; chlorinated rubbers; etc. Furthermore, polyethylene filmtreated by the present process is more readily adhered to itself andother base materials by using com mercial adhesives, e. g., standardadhesives employed for sealing cellophane.

The process of this invention may also be employed for treating thesurface of various films fabricated from copolymers of ethylene withvarious other polymerizable i materials, e. g., isobutylene, vinylacetate, styrene, vinyl chloride.

The outstanding advantage of the present process is that it provides areadily applicable and rapid method of improving the adhesion of a driedprinting ink to the surface of a polyethylene film. The process may bereadily combined with a necessary step of extruding or calenderingmolten polyethylene into film or tube form, and the additional apparatusrequired is inexpensive and easy to install.

An many widely different embodiments may be made without departing fromthe spirit and scope of this invention, it is to be understood that saidinvention is in no wise restricted except as set forth in the appendedclaims.

I claim:

1. A process for treating structures of polyethylene which comprisessubjecting said structures to the action of ozone at a temperature of atleast 150 C., quenching said structures in water containing aconditioning agent selected from the group consisting ofhydrogen-peroxide, nitrous acid, alkaline hypochlorites, concentratednitric acid and mixtures of concentrated nitric acid and concentratedsulfuric acid, and thereafter neutralizing said structures in an aqueousalkaline bath.

"'6 t 2. Theprocess of claim 1 wherein the action ofozone is carried outin-the presence of ultra-violet light having a wave length no greaterthan 3900 A. U. i

3. A process for treating structures of polyethylene which comprisessubjecting said structures to the action of ozone at a temperaturewithin the range of from about toabout 325 C., quenching said structuresin water containing a conditioning agent selected from the groupconsisting of hydrogen peroxide, nitrous acid, alkaline hypochlorites,concentrated nitric acid and mixtures of concentrated nitric acid andconcentrated sulfuric acid, and thereafter neutralizing said structuresin an aqueous alkaline bath.

4. The process of claim 3 wherein the action of ozone is carriedout inthe presence of ultra-violet light having a wave length no greater than3900 A. U.

5. A process for treating polyethylene film which comprises subjectingthe surface of said film to the action of ozone at a temperature'of atleast 150 C., quenching the film in water containing a conditioningagent selected from the group consisting of hydrogen peroxide, nitrousacid, alkaline hypochlorites, concentrated nitric acid and mixtures ofconcentrated nitric acid and concentrated sulfuric acid, and thereafterneutralizing the film in an aqueous alkaline bath.

6. Polyethylene film when treated in accordance with the process ofclaim 5.

7. A process for treating polyethylene film which comprises subjectingthe surface of said film to the action of ozone at a temperature withinthe range of from about 150 to about 325 C., quenching the film in watercontaining a conditioning agent selected from the group consisting ofhydrogen peroxide, nitrous acid, alkaline hypo- 'chlorites, concentratednitric acid and mixtures of concentrated nitric acid and concentratedsulfuric acid, and thereafter neutralizing the film in an aqueousalkaline bath.

8. The process of claim 7 wherein the action of ozone is carried out inthe presence of ultra-violet light having a wave length no greater than3900 A. U.

9. A process for treating polyethylene film which comprises passingcontinuous polyethylene film continuously through a zone wherein saidfilm is maintained at a temperature of at least 150 C., subjecting thefilm in said zone to the action of ozone, continuously quenching saidfilm in water containing a conditioning agent selected from the groupconsisting of hydrogen peroxide, nitrous acid, alkaline hypochlorites,concentrated nitric acid and mixtures of concentrated nitric acid andconcentrated sulfuric acid, and thereafter neutralizing the film in anaqueous alkaline bath.

10. A process for treating polyethylene film which comprises passingcontinuous polyethylene film continuously through a zone wherein saidfilm is maintained at a temperature within the range of from about 150to about 325 C., subjecting the film in said zone to the action ofozone, continuously quenching said film in water containing aconditioning agent selected from the group consisting of hydrogenperoxide, nitrous acid, alkaline hypochlorites, concentrated nitric acidand mixtures of concentrated nitric acid and concentrated sulfuric acid,and thereafter neutralizing the film in an aqueous alkaline bath.

11. The process of claim 10 wherein the action of ozone is carried outin the presence of ultra-violet light having a wave length no greaterthan 3900 A. U.

12. A process for treating polyethylene film which comprises passingcontinuous polyethylene film continuously through a zone wherein saidfilm is maintained at a temperature within the range of from about 150to about 325 C., subjecting the film in said zone to the action of agaseous atmosphere containing at least 0.05% by volume of ozone as theactive ingredient, continuously quenching said film in water containinga conditioning agent selected from the group consisting of hydrogenperoxide, nitrous acid, alkaline hypochlorites, concentrated nitric acidand mixtures of concentrated nitric acid and concentrated sulfuric acid,and thereafter neutralizing the film in an aqueous alkaline bath.

13. The process of claim 12 wherein the water is maintained at atemperature of from about 30 to about 65 C.

14. In the process wherein molten polyethylene is continuously formedinto film and the freshly formed film at a temperature of from 150 to325 C. is continuously passed through a water quench bath, theimprovements which comprise subjecting the film prior to quenching tothe action of ozone, and incorporating in the quench bath a conditioningagent selected from the group consisting of hydrogen peroxide, nitrousacid, alkaline hypochlorites, concentrated nitric acid and mixtures ofconcentrated nitric acid and concentrated sulfuric acid, andneutralizing the quenched film in an aqueous alkaline bath.

15. A process for treating polyethylene film which comprises subjectingthe surface of said film to the action of ozone at a temperature of atleast 150 C., quenching the film in watercontaining a conditioning agentselected from, the group consisting of hydrogen peroxide, nitrous acid,alkalinehypochlorites, concentrated nitric acid and mixtures ofconcentrated nitric acid and concentrated sulfuric acid, neutralizingsaid film in an aqueous alkaline solution, and thereafter imprintingsuch surface with a printing ink.

References Cited in the file of this patent UNITED STATES PATENTS2,400,720 Staudinger May 21, 1946 2,499,421 Samler Mar. 7, 19502,502,841 Henderson Apr. 4, 1950 2,612,480 May Sept. 30, 1952 2,622,056DeCoudres Dec. 16, 1952 2,639,998 Pavlic May 26, 1953

1. A PROCESS FOR TREATING STRUCTURES OF POLYETHYLENE WHICH COMPRISESSUBJECTING SAID STRUCTURES TO THE ACTION OF OZONE AT A TEMPERATURE OF ATLEAST 150*C., QUENCHING SAID STRUCTURES IN WATER CONTAINING ACONDITIONING AGENT SELECTED FROM THE GROUP CONSISTING OF HYDROGENPEROXIDE,